Synthetic lubricants are generally specified in demanding high value applications such as stationary turbines, jet engines, hydraulic systems, and the like. Some synthetic lubricants are known as “polyol esters” and include compounds formed from monobasic fatty acids and polyhydric alcohols having a “neopentyl” structure. Representative alcohols useful for forming synthetic ester lubricants include neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol. These alcohols are reacted to form esters with fatty acids generally having from about five to about twelve carbon atoms including: valeric, isopentanoic, hexanoic, heptanoic, octanoic, isooctanoic, 2-ethylhexanoic, pelargonic, isononanoic, decanoic and dodecanoic. The alcohols listed above generally have no beta-hydrogens and differ primarily in the number of hydroxyl groups available to form esters.
Depending on the fatty acids selected, i.e., same or different and numbers of carbon atoms, the properties of the polyol ester formed can be “designed” to provide a particular viscosity range, pour point, flash point and volatility as required for particular applications. Lower molecular weight acids, e.g., valeric, isopentanoic, etc., generally are used when flowability at low temperatures is important. Properties such as oxidative stability and resistance to hydrolysis may be enhanced by incorporation of acids having branching. In many applications, mixtures of both higher and lower molecular weight acids provide desirable properties. U.S. Pat. No. 4,440,657 discloses many simple esters, diesters and polyol esters suitable for use as lubricants.
In addition to selecting a synthetic lubricant based on its chemical structure, various additives are blended into the lubricant to enhance its oxidation resistance, disperse sludge formed, improve hydrolysis resistance, passivate metals, inhibit rust and the like. Antioxidants formed from polymerizing, for example, alkylated diphenyl amines and alkylated phenyl-α-naphthyl amines are widely used in many industrial applications including, but not limited to, thermoplastic resins, lubricants and hydraulic fluids to improve resistance to oxidation. U.S. Pat. No. 3,509,214 discloses that aromatic secondary naphthyl amines or N-arylnaphthylamines may be coupled or cross-coupled to form oligomers which, when present as additives in synthetic lubricants, enhance the resistance of the lubricants to high temperature oxidation.
Additional representative antioxidant additives are disclosed in U.S. Pat. No. 5,160,647 which discloses formaldehyde condensation products of alkylphenyl substituted-1-aminonaphthalenes. In this patent, all of the preparations of the disclosed compounds are prepared in solvent with the compound being isolated and purified as a solid material.
U.S. Pat. No. 3,492,233 discloses lubricant compositions containing dehydrocondensation products produced by chemically attaching an additive molecule such as an antioxidant, load-carrying agent, detergent, anticorrosion agent and the like to a lubricant molecule by heating the additive and an organic lubricant base fluid in the presence of an organic peroxide.
U.S. Pat. No. 6,426,324 discloses an antioxidant composition suitable for ester fluid lubricants formed from diphenylamines and N-aryl naphthylamines in the presence of an organic peroxide. In this disclosure, a reaction with a polyol ester lubricant base is disclosed, but is, according to the disclosure, a byproduct and ways to minimize the reaction between the amines and the polyol ester fluid are preferred.
Present preparations of antioxidants for lubricant fluids are multi-step and often require difficult and expensive separations and purifications. These current preparations add to the cost of preparing already expensive synthetic lubricants, such as polyol esters. If a compound having suitable antioxidant properties were available and a less complex, more efficient method for its preparation and incorporation into synthetic lubricants was provided, the art of stabilized polyol ester lubricants would be enhanced. Such a compound and a method for its preparation and incorporation into a synthetic polyol ester lubricant are provided herein below.